剑指 Offer 54. 二叉搜索树的第k大节点

解法一

迭代

class Solution {
public:
    int kthLargest(TreeNode* root, int k) {
        stack<TreeNode*> stc;
        int n=0;
        while (!stc.empty() || root)
        {
            while (root)
            {
                stc.push(root);
                root=root->right;
            }
            root=stc.top();
            stc.pop();
            n++;
            if(n==k) return root->val;
            root=root->left; 
        }
        return 0;
    }
};

解法二

递归，性能更优

class Solution {
public:
    int res=0;
    int n=0;
    int kthLargest(TreeNode* root, int k) {
        dfs(root,k);
        return res;
    }
    void dfs(TreeNode* root, int k){
        if(root==NULL) return ;
        if(root->right) dfs(root->right,k);
        n++;
        if(n==k) res=root->val;
        if(root->left) dfs(root->left,k);
        return ;
    }
};